1. Field of the Invention
The present invention relates to a motor, and more particularly to a motor generating vibrations.
2. Description of the Related Art
A vibration motor of this type has been conventionally used in a selective calling receiver and others. The vibration motor in the selective calling receiver is used for informing a user of a call reception.
A conventional vibration motor is provided with a motor body having a rotary shaft. Further, a weight is attached to the rotary shaft. The weight is so disposed as to be eccentric with respect to the rotary shaft.
In the conventional motor, the rotary shaft is rotated with the electric power supplied from a motor power supply terminal. The rotation of the rotary shaft involves the rotation of the weight, and the vibration motor generates vibrations.
In the conventional vibration motor, however, since the weight is directly fixed to the rotary shaft, a large starting torque for rotating the weight is required. The motor power supply terminal must therefore supply a large amount of current to the motor body, and hence the selective calling receiver using the motor of this type requires a large scale power supply. The dimension of the selective calling receiver thus becomes large, deteriorating the portability.
U.S. Pat. Nos. 4,830,534 and 3,822,604, to Schmelzer et al. and Grimmer, respectively, disclose vibration motors having a weight which is displaced from a rotary shaft of the motor when the shaft rotates to provide an eccentricity and thus a vibration. While these inventions have their benefits, the shape of the weights used leads to inefficiencies in the required starting torques. Also, the shape of the weight is such that it does not contribute efficiently to the eccentricity of the system when the weight is fully displaced from the shaft.
In addition, as a prior art vibration motor, a vibration motor using ultrasonic vibrations produced by a piezoelectric device as a driving source has been also used. This type of motor is disclosed in Japanese Patent Laid-open No. heisei 2-111269 or Japanese Patent Laid-open No. showa 63-262069, for example.
This type of motor has however a complicated structure, thereby increasing the manufacturing cost.